The basic conventional telephone intercom system normally comprises a single link which is accessible by any station at all times and wherein it is not possible to exclude a third party after a conversation has been established between a calling first party and a called second party. Also, in such a conventional system it is usually difficult to exclude an additional station from the link after a conference call has been initially established between three or more stations in the one-link system.
A primary drawback to the common talk-path system of the prior art is that it is limited to handle a single intercom conversation or conference at any one time. Multiple talk-path systems which now exist have the disadvantage that they either require multiple dial-selective intercom units (and use more than one button on the associated key telephone) or else require a separate talk battery for each station phone (similar in function to the A-relay of a central office), as in the Western Electric 6A system. It would be highly desirable to provide a second talking path in a simpler manner.
Thus, there is a need for a suitable means for converting a one-link non-exclusive intercom system into a two-link system wherein a private link is included, with means for excluding additional calls from the private link, as above described, and for automatically transferring a conversation or conference into said private link when it becomes available.